The present invention relates to injection molding machines and relates in particular to product retrieval from continuously operating, single or multi-cavity molding machines.
The prior art is replete with various methods and devices (robots) for entering an open mold to pick up molded articles in synchronism with a continuous molding cycle.
Most of the prior art units are massive. Therefore, they are difficult to accelerate and decelerate as the units reciprocate to and from into and out of the mold.
The time required for the robot to enter and exit from the mold area directly effects cycle time since the injection molding machine is "waiting" for the robot to clear the area so the molding cycle can continue. Therefore, faster part removal devices offer significant advantages relative to increase productivity.
It is necessary to maintain accurate registration between the article retrieval head and the pattern of the mold so that when the transfer of molded product from the mold to the retrieval unit occurs each individual molded article is "picked-up" cleanly without distortion and each cavity is emptied.
Vibration of retrieval units frequently results in failure to pick the mold clean or in distortion or mutilation of some of the molded articles.
Furthermore in massive retrieval units it is difficult to operate the units as high speeds in synchronism with molding operations.
Efforts to operate massive retrieval units at high speeds tends to accentuate the vibration problem which is caused by inadequate stiffness relative to the mass of the unit. Therefore, frequently it is necessary to introduce a dwell period at the end of a stroke to permit damping of the vibrations.
The time required for a robot to travel from one point to another is largely dependent on how quickly the robot can reach its maximum velocity and how quickly it can stop; in other words how fast it can accelerate and decelerate. An undesirable consequence of accelerating too quickly is that it can create or accentuate vibration resulting in inaccurate positioning relative to the mold. Acceleration and deceleration requires the input or removal of energy from the unit in terms of a force acting on the components that need to be moved. The greater the mass that must be moved, the greater the forces and therefore, energy that is involved. These forces must be dealt with and it is the reaction of these forces acting to deform the bodies that result in vibration. The stiffness of the components determines the resulting displacement associated with the energy (forces) causing the vibration.
Vibration has normally been reduced by making components larger (in order to make them stiffer) so that the vibrations result in smaller movements relative to the mold. This, in turn, adds to the vibration problem since more mass now needs to be moved. The ideal situation is one where there are very stiff components that have very little mass and it is the objective of this invention to meet this criteria. Light (small mass) components and assemblies can be accelerated very quickly without inducing any substantial vibration because they have relatively low inertia.
Inertia can be defined as the property of a body that maintains it in the state of rest or motion until acted on by some force with its mass being a measure of inertia.
Momentum is defined as the property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force and is expressed mathematically as the product of mass and velocity for a body moving in a linear motion in given direction. Changing either the mass, direction or velocity will change the momentum.